Rotary lawn mowers as contemplated herein comprise a housing generally surrounding a cutting blade mounted for rotary movement therein in a horizontal plane. For reference purposes the housing may be considered to be divided in plan into a 360.degree. interval, commencing where the forward facing cutting edge of the cutting blade is coincident with the central transverse axis of the lawn mower and incrementing in the direction of rotation of the cutting blade.
The cutting zone of the lawn mower is located in the angular interval 0.degree.-180.degree., the cutting action being at a maximum at 90.degree.. In conventional rotary lawn mowers of a first type the grass cuttings are discharged through a discharge opening provided in a side wall forming part of the housing located adjacent the end of the cutting zone and extending over an arc of about 30.degree.. In rotary lawn mowers of a second type the grass-cuttings are discharged through a discharge opening provided in the top wall of the housing, and extending over the last part of the cutting zone, generally in the angular interval of about 90.degree. to 180.degree.. A discharge chute connects to the top discharge opening to conduct the grass cuttings to a collector bag located at the rear of the lawn mower.
It is well known to provide combination rotary lawn mowers which, by the use of suitable adapters, may be converted from one of the aforesaid types to the other. Such conversion poses relatively little problem, as the air flow patterns associated with the two types are not too dissimilar, with air being drawn into the housing from beneath the outer periphery thereof and expelled with the entrained cuttings through the discharge opening. Generally the cutting blade is adapted to function as an air propeller. Such cutting blade is normally planar, with each end thereof having an upturned portion so as to provide an upward air propelling surface.
More recently it has been proposed to provide rotary lawn mowers without discharge openings. Generally in such lawn mowers, grass cut in the angular interval 0.degree.-180.degree. is comminuted by the cutting blade, preferably in the angular interval 180.degree.-360.degree. so as to equalize the load on the blade, to form a fine mulch for deposition onto the mown grass surface beneath the housing. The air flow patterns within the housing differ appreciably in the chuteless rotary lawn mowers from those obtaining in rotary lawn mowers with discharge openings, generally requiring a cycling air flow to suspend and transfer the cuttings. Two somewhat different proposals for effecting mulching may be referenced. In U.S. Pat. No. 3,608,291, Kidd, dated Sept. 28, 1971 the housing has a top wall which is generally uniformly upwardly inclined over the angular interval of about 0.degree.-180.degree., and generally uniformly downwardly inclined over the angular interval of about 180.degree.-270.degree.. Whilst the cutting blade is not shown, it may be understood to be of the aforedescribed type, i.e. generally planar with an upward air propelling surface adjacent each end thereof. The suspended cuttings are said to move upwardly in the expanded volume of the first 180.degree. interval, to be deflected downwardly over the interval 180.degree.-270.degree. by the topwall, and intersect the blade to be comminated thereby in the interval 270.degree.-360.degree.. In U.S. Pat. No. 3,085,386, Slemmons, dated Apr. 16, 1963, a mulching lawn mower is described wherein the cutting blade and blade housing cooperate to provide a helical air flow pattern within the blade housing. The blade housing in such proposal is described as being semi-toroidal and of constant cross section so as to avoid any pressure changes within the housing. The cutting blade is provided with an upwardly inclined air directing surface adjacent the tip thereof, and a downwardly inclined air directing surface intermediate the axis of the cutting blade and the upwardly inclined air directing surface. This type of construction tends to maintain the grass cuttings in suspension whereby they may be subject to comminution by the cutting blade several times.
It is found that the capacity of mulching type lawn mowers to comminute grass cuttings to a fine mulch which may be dispersed into a mown lawn surface with aesthetically pleasing results is somewhat limited. In the early cutting season, when the grass growth is very vigorous and the moisture content is high, the cutting capacity is readily overloaded. It is therefore desirable to provide in a lawn mower the capability of discharging the cuttings from the blade housing prior to their being further comminuted to any appreciable extent so as to reduce the load on the cutting blade. The air flow pattern within the housing of mulching lawn mowers particularly of the second type employing a helical air flow path is not conducive to the efficient discharge of grass cuttings, and it is not believed that a practical arrangement has been known heretofore for a rotary lawn mower which may be optionally employed for mulching operations or for operation wherein grass cuttings are discharged through a side discharge or a top discharge chute as desired.